With the development of wireless communication, can be widely applied on many fields and provide potential business market opportunities thus it is always a hot topic, for example, the global satellite position, back car radar, directional car search, people searching, or object searching etc. The positioning technique is particularly useful for the team tours in mountain if every team member tied with or bring a transceiver. One of positioning techniques may refer to the RSSI (received signal strength indicator) to derive the distance of the object or people. The RSSI position technique involves using the RFID and the antennas. To position an object in a positioning space, four antennas are demanded or in a 2D plane by three antennas. FIG. 1A, a schematic diagram, depicts the 2D planar positioning. Three antenna readers are assumed placed at positions A, B, C and a target to be positioned is assumed at the point P. If the target has a transmitter to transmit a wireless signal then the antenna readers A, B, and C would receive the signal thereby the distances dpA, dpB, dpC can be estimated according to the RSSI values detected. Accordingly, the coordinate of the P can be obtained by solving the coupling equations:dpA=√{square root over ((xA−xP)2+(yA−yP)2)}{square root over ((xA−xP)2+(yA−yP)2)}  (1)dPA=√{square root over ((xB−xP)2+(yB−yP)2)}{square root over ((xB−xP)2+(yB−yP)2)}  (2)dpC=√{square root over ((xC−xP)2+(yC−yP)2)}{square root over ((xC−xP)2+(yC−yP)2)}  (3)
However, the RSSI values are vulnerable to be affected by environmental factors unless the RSSI values have been passed a long training time and/or correctness thereafter.
Another positioning method of the radio signal positioning is by time arriving, which is found to be more precisely and less be affected by environment than RSSI. Please refer to FIG. 1B, a first signal is sent out at position A by a first transceiver, which cost a duration tdur. Upon receiving the first signal by the second transceiver at time tB at the position B, the second transceiver transmits a second signal back immediately and received by the first transceiver at time tA.
The distance dAB between position A and position B can be expressed as:
            d      AB        =                                        t            A                    -                      t            B                          2            ×      C            where    ⁢                  ⁢    C    ⁢                  ⁢    is    ⁢                  ⁢    the    ⁢                  ⁢    velocity    ⁢                  ⁢    of    ⁢                  ⁢          light      .      
In practice, both of the wireless signals are modulated signals transmitted according to a data packet protocol, which are a baseband signal modulated by a carrier signal. After the modulated signal received by the transceiver, the modulated signal is demodulated to return to the baseband signal. To position by using a wireless signal technique, the crystal oscillation frequencies of the first transceiver and the second transceiver should be in consistence; otherwise, the arrival time would be incorrect.
Unfortunately, even using the most advanced semiconductor processes, two crystal oscillators are generally found to have a frequency offset. The frequency offset is usually negligible, e.g. one or several ppm. (parts per million). However, the distance error caused by the frequency offset will be not negligible but significantly due to the value that the frequency offset times velocity of light is not negligible. Thus if there is any frequency offset, it will be inferior to use in positioning.
Consequently, it is important to know the frequency offset between two oscillation frequencies and further cancel them before using them in frequency synthesizer to generate a carrier signal.
A conventional but exemplary frequency synthesizer is disclosed by an U.S. Pat. No. 7,649,428 having a title “Method and System for Generating Noise in a Frequency Synthesizer.” The patent is to generate a noise portion of an input signal within the frequency synthesizer and appending the noise portion the noise portion to a control portion of the input signal. The functional blocks diagram, please see the FIG. 1C. The frequency synthesizer is a fractional-N synthesizer with a PLL (phase locked loop) 20, a sigma delta modulator 32, and a mixer 34. The phase locked loop 20 includes a phase frequency detector (PFD) 22, a charge pump 24, a loop filter 26, a voltage-controlled oscillator 28, a multi-modulus divider 30 in series connected to compose a loop. The output signal Fout of the VCO 28 is fed back to the multi-modulus divider 30 and the latter then outputs a signal and is compared with an input signal FIN by the phase detector 22.
The sigma-delta modulator 32 outputs a fractional part Δ[K], which is then summed with an integer N by a summer 34. Accordingly, the summer 34 provides a fractional integer, represented by “N.Δ[K]” as a divisor to the multi-modulus divider 30. As a result, the frequency Ferr is equal to Fout divided by (N.Δ[K]).
The frequency Ferr is then compared with the FIN of the input signal of the PLL circuit 20. If the phase difference between two input signals Ferr, FIN of the PFD 22 is over ±2π then the PFD 22 is operated in the frequency detect mode, then the charge pump 24 operated is in full speed, i.e., running a constant current. The full speed operating process of the charging pump 24 is continuously until the phase difference between two input signal drops within ±2π.
The PFD is thus running into a phase detect mode. The output of the charge pump 24 is proportional to the phase difference. Once the phase difference of two signals reaches zero. The device enters into the phase locked state.
The prior art doesn't relate to a method of correcting the frequency offset between two transceivers at two locations.
An object of the present invention thus provides a frequency synthesizer having frequencies outputted tracing the signal received so that the frequency offset of two frequency synthesizers of the transceivers can be cancelled.